Phosphate ores occur in important deposits in various parts of the world, including central Florida. Each deposit has characteristic impurities which must first be removed to increase the phosphate content of the material so that it can be used in fertilizers to enrich farmlands and thereby increase crop yields. The phosphate content of fertilizers are generally expressed as "BPL" content [bone phosphate of lime, or Ca.sub.3 (PO.sub.4).sub.2 ].
The phosphate ore deposits found in central Florida generally contain siliceous mineral (quartz) impurities, and the lower zones of some of these deposits also contain carbonate mineral impurities including dolomite [Ca,Mg]CO.sub.3. Such phosphate ores have been improved in phosphate content by various "beneficiating" processes to remove a major portion of the impurities, and thereby increase the phosphorus content [expressed in terms of phosphorus pentoxide (P.sub.2 O.sub.5) by the chemist].
Froth flotation and skin flotation beneficiation are conventionally used to remove siliceous gangue materials from the phosphate-containing ores. In such a process, the ore materials are classified into various particle sizes. The coarser fractions may be suitable for direct sale, or may be further beneficiated by sizing and by froth and skin flotation techniques. The very fine materials, which primarily contain clay slimes and clay-sized particles are usually discarded. The intermediate fraction, which typically has a particle size range from about 0.10 mm up to about 1.0 mm represents the bulk of the material which has the greatest need for beneficiation.
U.S. Pat. No. 2,293,640 issued to A. Crago describes a "double float" froth flotation which is commercially used for beneficiating such fractions of phosphate ores in which siliceous minerals (quartz) are the predominant gangue. The intermediate size fraction is conditioned with fatty acid reagents and the phosphate mineral is floated to separate it from the bulk of the silica tail impurities. The float portion is deoiled with sulfuric acid, rinsed, and refloated with amine reagents to float away the remaining silica tail impurities.
Removal of dolomite and similar carbonate mineral impurities from phosphate ores such as apatite, fluorapatite and francolite by the above double flotation method has generally been ineffective because the flotation characteristics of the carbonate minerals (dolomite) are very similar to those of these phosphate-containing minerals.
As used herein, "francolite" is intended to refer to sedimentary apatites, found in phosphate-containing mineral ores in Florida, including carbonate fluorapatite. These ores usually carry quartz (silica), and some contain dolomite [Ca,Mg]CO.sub.3 impurities. In the Florida phosphate deposits, "francolite" with dolomite impurities is typically found in the lower zone of the Hawthorn formation in which the carbonates of calcium and magnesium have not been leached out.
Phosphate ores containing primarily quartz impurities with lesser amounts of dolomite impurities are often found in an upper zone in a phosphate deposit above the lower zone. In the past, the practice has been to recover only the upper zone phosphate ores and leave the lower zone ores because the methods available to remove dolomite added too much cost to the product, and the added recovery was not considered worth the effort. More recently, however, the market value of phosphate fertilizers has increased sufficiently to warrant the added recovery costs of mining the lower zone deposits simultaneously with the upper zone phosphate ore deposits.
Methods for beneficiating phosphate ores containing carbonate and siliceous gangue materials are described in U.S. Pat. Nos. 3,259,242; 3,462,016; 3,462,017; 3,807,556; 4,144,969 and 4,189,103. Each of these patents deals with the difficulties associated with the separation of carbonate mineral impurities such as dolomite from the phosphate ore.
U.S. Pat. Nos. 4,144,969 and 4,189,103 which are assigned to a common assignee herewith, describe a phosphate ore beneficiating process in which the deslimed ore is first subjected to a "double float" froth flotation as described in U.S. Pat. No. 2,293,640 to remove siliceous gangue. The float product containing apatite with dolomite impurities is then conditioned at about 70% solids with an apatite-collecting cationic reagent and a liquid hydrocarbon, and then subjected to a froth flotation. Most of the apatite is recovered in the froth concentrate and the alkaline earth metal carbonate impurities (dolomite) are rejected in the underflow tailings.
Dolomite is removed as a float or froth concentrate from phosphate-containing ores in the processes described in U.S. Pat. Nos. 3,462,016; 3,462,017 and 3,806,556. Siliceous gangue is removed in the first stage of a two-stage anionic flotation. The siliceous gangue is the underflow, while the phosphate minerals with carbonate impurities are separated as the "float" in the first stage. The phosphate minerals are then selectively depressed for removal as an underflow concentrate, and the carbonate impurities are floated in the second stage. Fatty acid collector reagents such as oleic acid, stearic acid, and other carboxylic acids including tall oils are used for both the first and second stage anionic flotation. In the second stage, the carbonate particles are floated, and the apatite particles (phosphate) sink.
The Johnston U.S. Pat. No. 3,807,556 adds a soluble sulphate salt in the interstage conditioning of the above processes to reduce the loss of soluble phosphate in the second stage flotation. It is believed that the Johnston patent tacitly recognizes the sensitivity of the fatty acid collector reagents in the above flotation to variations of pH frequently experienced in actual plant operations. See Johnston U.S. Pat. No. 3,807,556, column 3, lines 3-16.
The general concept of carbonate flotation, however, is very desirable to obtain a greater yield of phosphate from phosphate ores containing both apatite and carbonate. In order to have a commercially practical process for carbonate flotation, the problems described above in the Johnston patent must first be solved.
A process for more effective carbonate separation has application to certain central Florida deposits and the western phosphates found in Idaho, Montana, Utah and Wyoming. Applicants have developed such a process which has particular utility for the dolomite-containing phosphate ores of central Florida.
The copending application of Robert E. Snow, U.S. Ser. No. 269,448 filed on even date herewith, is directed to a flotation method for beneficiating a phosphate ore concentrate to remove dolomite [Ca,Mg]CO.sub.3 by a reverse flotation with an anionic flotation agent comprising a sulfonated fatty acid.
As used herein, the term "mesh" refers to standard Tyler mesh, and if an ore fraction is said to have a particle size smaller than a certain mesh, such statement means that substantially all of the fraction will pass through a screen having that Tyler mesh size, and likewise, if an ore fraction is said to have a particle size greater than a certain mesh, then substantially none of the material will pass through a screen having that Tyler mesh size. As used herein, the symbol "M" also refers to Tyler mesh size.